This invention relates to a process for producing an aromatic polyamide film excellent in transparency which is used as a lens-protecting film or the like.
An aromatic polyamide, particularly all-para aromatic polyamide, is one of the polymers having the highest tensile modulus of elasticity among the heretofore known polymers.
A film obtained from the aromatic polyamide can be formed into a thin film having a thickness of several micrometers because of its very high tensile modulus, and also has excellent heat resistance because of its very high melting point or decomposition temperature.
However, the aromatic polyamide is dissolved in only special solvents having a high polarity such as con. sulfuric acid and the like and hence is difficult to handle. In addition, the aromatic polyamide has a lyotropic liquid crystallinity, so that only a devitrified, low-strength film containing a great number of crystallite is obtained by a conventional solution-casting method.
As a process for producing a transparent aromatic polyamide film, JP-A-62-246,719 discloses a process comprising separating as a polymer an aromatic polyamide obtained by polymerization in a solvent such as N-methyl-2-pyrrolidinone or the like, subsequently dissolving the polymer in con. sulfuric acid, moistening the resulting optically anisotropic, con. sulfuric acid solution at a high temperature to make it optically isotropic, subjecting the same to coagulation and washing, and then heat-treating the resulting film at a temperature of 300.degree. to 500.degree. C. under tension. However, said process has many steps and requires a complicated operation. In addition, the equipment to be used must be resistant to sulfuric acid. Therefore, there is such a problem that the cost becomes very high.
The present inventors have made extensive research for solving the above problems and have, as a result, found that a transparent film is obtained by forming a thin layer of an optically isotropic solution in which an aromatic polyamide is dissolved in a polar amide (this solution is hereinafter referred to as the dope in some cases), thereafter immersing the thin layer dope in a low-temperature polar solvent and then drying the same.